Best demo I've seen is the early electric submarines where batteries were switched between "Group up" and "Group down", short and fast vs long and slow.

John:

Were they controlling the speed of the motors with this series/parallel approach?

In the old days, diesel trucks sometimes had 24 volt starters and four 6 volt batteries that were charged as 2 pairs of 6 volt wired in series. To start they were all in series.

Another example of a series/parallel device is the Marx Bank, these are used to generate high voltage pulses, they are charged in parallel and discharged in series. We have some at work, 150 KiloVolt and 450 KiloVolt units used to produce flash X-rays.